The present invention relates to a method for quantitatively assaying coagulation factor XII (Hageman factor) in human plasma by activation of factor XII (proenzyme), present in the plasma, to factor XIIa (enzyme), and directly reacting the latter with a substrate capable of being split enzymatically.
Factor XII is the first factor which is activated in the endogenic blood coagulation cascade. Owing to the fact that this factor plays a key role not only in the coagulation system but also in the kallikrein-kinin system and in the fibrinolytic system, it is important to have means for quantitatively assaying this factor in blood plasma. The assaying method most frequently used consists in incubating a plasma sample with kaolin in order to cause maximum activation of the Hageman factor, incubating the activation mixture with plasma deficient in Hageman factor, recalcifying the incubation mixture after a predetermined time by the addition of calcium ions and measuring the coagulation time of the recalcified mixture. The coagulation time is approximately a function of the concentration of Hageman factor in the test sample (see e.g. Oscar D. Ratnoff in "Thrombosis and Bleeding Disorders", 1971, Georg Thieme Verlag, Stuttgart, Germany, pages 215-218). The disadvantage of this assaying method resides in the fact that one measures a value which results from a long cascade of reactions and hence is dependent on a large number of reaction parameters. An additional difficulty results from the fact that plasma deficient in Hageman factor needed for establishing calibration curves is scarcely available.